Preservation of corneal integrity by the mucin secretagogue 15(S)-HETE in a rabbit model of desiccation-induced dry eye

Rabbit models of dry eye disease: Current understanding and unmet needs for translational research

Dry eye disease (DED) is emerging as an eye health pandemic, affecting millions worldwide. The development of novel drugs, drug delivery systems, and targeted therapies for addressing the inflammation in DED necessitates progress in experimental models of DED. Animal models of DED have been created for simulating the two clinically described forms of DED: lacrimal insufficiency and the evaporative DED models. Although most DED models have relied upon rodents, the larger eye size and longer life span of rabbits and the closer resemblance to human lacrimal glands, render rabbits a promising near-ideal model for studying DED. Since the first rabbit DED model was described, numerous modifications including the use of topical epitheliotoxic drugs, neural abolition, activated lymphocytes injection, and surgical dacryoadenectomy have been introduced. The stability of these models, whether short-term or long-term, accordingly guides their experimental or therapeutic utility. A rabbit autoimmune dacryoadenitis model has successfully simulated DED signs and features of lacrimal gland inflammation, as observed in Sjogren's syndrome, that improved with mesenchymal stem cell therapy. This review summarizes the comparative microscopic anatomy of rabbit and human lacrimal glands, various existing rabbit DED models and their respective suitability for understanding pathogenetic mechanism of DED or for experimental drug testing. Also, the insights gained from animal models in dry eye management is described along with the future perspectives. There is still a pressing need of developing rabbit models for studying the pathogenesis of complex ocular surface changes in evaporative and aqueous deficiency DED other than autoimmune dacryoadenitis.

Age-related Defects in Ocular and Nasal Mucosal Immune System and the Immunopathology of Dry Eye Disease

Dry eye disease (DED) is a prevalent public health concern that affects up to 30% of adults and is particularly chronic and severe in the elderly. Two interconnected mechanisms cause DED: (1) an age-related dysfunction of lacrimal and meibomian glands, which leads to decreased tear production and/or an increase in tear evaporation; and (2) an age-related uncontrolled inflammation of the surface of the eye triggered by yet-to-be-determined internal immunopathological mechanisms, independent of tear deficiency and evaporation. In this review we summarize current knowledge on animal models that mimic both the severity and chronicity of inflammatory DED and that have been reliably used to provide insights into the immunopathological mechanisms of DED, and we provide an overview of the opportunities and limitations of the rabbit model in investigating the role of both ocular and nasal mucosal immune systems in the immunopathology of inflammatory DED and in testing novel immunotherapies aimed at delaying or reversing the uncontrolled age-related inflammatory DED.

Reactive oxygen species activated NLRP3 inflammasomes initiate inflammation in hyperosmolarity stressed human corneal epithelial cells and environment-induced dry eye patients

In studies on dry eye (DE) disease, an association has been identified between tear film hyperosmolarity and inflammation severity elicited through receptor-induced increases in proinflammatory cytokine and chemokine release. These immune reactions might be mediated by inflammasomes, macromolecular complexes mounted around the NLRP3 protein and can be activated by reactive oxygen species (ROS) over-generation. Hence in this study we determine whether: a) ROS activated NLRP3 inflammasomes mediate hyperosmotic stress-induced inflammation in human corneal epithelial cells (HCECs); b) the ROS-NLRP3-IL-1β axis activation is associated with environment-induced DE. Immortalized HCECs were exposed to 500 mOsm medium in the presence and absence of a ROS inhibitor, N-acetyl-l-cysteine (NAC). HCECs transfected with NLRP3 siRNA or a negative control (NC) siRNA. Intracellular ROS was measured by fluorometric analysis using the probe 2',7'-dichlorofluorescin diacetate (DCFH-DA). Real-time PCR evaluated NLRP3, ASC, pro-caspase-1 and pro-IL-1β mRNA levels. Western blot analysis assessed NLRP3 protein expression whereas caspase-1 activity was determined with a fluorometric assay. Bioactive IL-1β release was assessed by ELISA. ROS production, NLRP3 inflammasome and pro-IL-1β gene expression as well as IL-1β secretion were also evaluated in the conjunctival epithelial cells and tear fluid samples of environment-induced DE patients and normal subjects. NAC suppressed hyperosmolarity-induced rises in ROS levels, NLRP3 inflammasome formation and activation, caspase-1 activity and IL-1β release. On the other hand, NLRP3 siRNA knockdown inhibited hyperosmotic stress-induced NLRP3 activation, which led to ASC, pro-caspase-1 and pro-IL-1β mRNA down-regulation followed by suppression of associated caspase-1 activity and IL-1β secretion. In addition, in ocular surface samples of environment-induced DE patients, ROS generation, NLRP3, ASC, pro-caspase-1 and pro-IL-1β gene expression as well as IL-1β secretion were upregulated. Taken together, NLRP3 mediated innate immune responses triggered by rises in ROS generation induce inflammation in hyperosmotic stressed HCECs. ROS-NLRP3-IL-1β signaling pathway might play a priming role in environment-induced DE development.

Reactive oxygen species activated NLRP3 inflammasomes prime environment-induced murine dry eye

Tear film hyperosmolarity along with exposure to oxidant stress are factors that can induce chronic ocular surface inflammation and pain. However, there is limited information on how increases in reactive oxygen species (ROS) generated by oxidant exposure can induce inflammation. There is emerging evidence in other tissues that innate immune responses to a variety of environmental stresses stem from ROS-induced cytosolic NLRP3 inflammasome activation. Once this occurs, pro-caspase-1 is converted into its catalytic active form, which in turn cleaves pro-IL-1β thereby generating its bioactive form. We determined the role of ROS generation in mediating increases in IL-1β secretion through caspase-1 activation caused by NLRP3 inflammasome activation in an environment-induced murine dry eye (DE) model. An intelligently controlled environmental system (ICES) induced evaporative DE in female 4-6 week old C57BL/6J mice. Increases in ROS production preceded rises in corneal and conjunctival gene expression of NLRP3 inflammasome components and IL-1β that were identified using real-time PCR. Confocal microscopy evaluated concomitant increases in NLPR3, caspase-1 and IL-1β immunostaining. Increases in caspase-1 activity were used as an indicator of inflammasome activation. Rises in ROS generation occurred after 1 week of ICES exposure, which preceded increases in gene expression of three NLRP3 inflammasome components (i.e. NLRP3, ASC and caspase-1) leading to rises in bioactive IL-1β release. Increases in caspase-1 activity occurred after 2 weeks of ICES exposure. Eyedrops containing 0.3% N-acetyl-l-cysteine (NAC) were applied to quench ROS generation by mice kept in the ICES for 2 weeks. This scavenger reduced corneal fluorescein staining and decreased ROS production. NAC also down-regulated both increases in NLRP3, ASC, caspase-1 and IL-1β mRNA levels, along with their immunostaining. It robustly attenuated rises in inflammasome mediated increases in caspase-1 catalytic activity. We show in a dessicating DE disease murine model that rises in ROS generation trigger NLRP3 inflammasome complexation and activation leading to increases in bioactive IL-1β secretion. These results prompt us to suggest that the ROS-NLRP3-IL-1β signaling pathway might play a priming role in environment-induced DE progression. Finally, our findings provide a basis for developing novel strategies that may improve the management of patients requiring treatment for environment-induced dry eye disease.

Effectiveness of dry eye therapy under conditions of environmental stress

Purpose: Dry eye is often characterized by increased tear evaporation due to poor tear film quality, especially of the lipid component of the tear film. Using an environmental chamber to induce environmental stress, this study compared the effect of three lubricant eye drops on various aspects of tear physiology in a crossover design (evaporation was the principal outcome measure).

Methods: Three eye drop formulas were tested: 0.5% carmellose sodium (Drop C), 0.5% carmellose sodium with added lipid (Drop C-L) and 1.0% glycerine with added lipid (Drop G-L). Nineteen control and 18 dry eye subjects used each product for 2 weeks, three times per day, in a random order, with a minimum 1-week washout between treatment periods. Tear evaporation, break up time, osmolarity, tear structure (by interferometry) and patient symptoms were assessed with the subjects adapted for 10 min in an environmental chamber controlled at 20% relative humidity and 22 °C. The treatment effects were analyzed using general linear model repeated measures analyses of variance.

Results: In dry eye subjects, evaporation, break up time, osmolarity and symptoms improved for all formulas (p < 0.05). Normal subjects showed some improvements: evaporation with C-L, osmolarity with C and symptoms with C-L and G-L. Change in evaporation was greater for both C-L and G-L versus C (p < 0.05), and there was a trend for C-L to reduce evaporation more than G-L (p < 0.11). There were no significant treatment effects on tear film structure.

Conclusion: Overall, the eye drop formula containing both carmellose sodium and lipid (C-L) produced a greater treatment effect on tear evaporation than the other formulations containing only one of these ingredients. This study also demonstrates the utility of a controlled environmental chamber in showing the difference in performance between dry eye treatments.

Effect of airflow exposure on the tear meniscus

Purpose. To compare the effect of airflow exposure on the tear meniscus and blink frequency in normal and evaporative dry eye subjects. Methods. In 9 normal subjects and 9 short tear breakup time (SBUT) dry eye subjects, lower tear meniscus height (TMH) and area (TMA) and blink frequency were measured with anterior segment optical coherence tomography (OCT) before and after 5 minutes of airflow exposure (1.5 ± 0.5 m/s). Results. In SBUT dry eyes, both TMH and TMA decreased significantly (P = 0.027, P = 0.027) with a significant increase of blink frequency after airflow exposure, while significant increase in TMA was found in normal eyes. Conclusion. Measurement of the tear meniscus with anterior segment OCT seems to be useful as a noninvasive and objective method for evaluating the effect of airflow on tear film.

Predicted phenotypes of dry eye: proposed consequences of its natural history

This paper reviews current knowledge of the pathophysiology of dry eye and predicts that the clinical picture in late disease differs in both severity and quality from that in early disease. It is hypothesized that hybrid forms evolve, in which aqueous-deficient dry eye (ADDE) takes on features of evaporative dry eye (EDE) and vice versa. As a consequence, early and late forms may require different diagnostic criteria and respond to different therapeutic regimes. Tear hyperosmolarity plays a key role in the damage mechanism of dry eye, and ADDE is recognized to be a low-volume, hyperosmolar state. As ADDE advances, a progressive decrease in lacrimal secretion occurs, exacerbated by loss of the corneal reflex. This causes a decrease in tear volume, thinning of the aqueous tear film, and retarded spreading of the tear film lipid layer. The latter is hypothesized to cause an increase in evaporative water loss and an added evaporative component to the dry eye. Thus, in advanced disease, the hybrid state would be an organic ADDE, accompanied by a functional EDE in the absence of meibomian gland dysfunction. This functional EDE would respond to agents that expand the tear volume, restore corneal sensitivity, or provide an artificial tear film lipid layer.

Management and Therapy of Dry Eye Disease: Report of the Management and Therapy Subcommittee of the International Dry Eye WorkShop (2007)

The members of the Management and Therapy Subcommittee assessed current dry eye therapies. Each member wrote a succinct evidence-based review on an assigned aspect of the topic, and the final report was written after review by and with consensus of all subcommittee members and the entire Dry Eye WorkShop membership. In addition to its own review of the literature, the Subcommittee reviewed the Dry Eye Preferred Practice Patterns of the American Academy of Ophthalmology and the International Task Force (ITF) Delphi Panel on Dry Eye. The Subcommittee favored the approach taken by the ITF, whose recommended treatments were based on level of disease severity. the recommendations of the Subcommittee are based on a modification of the ITF severity grading scheme, and suggested treatments were chosen from a menu of therapies for which evidence of therapeutic effect had been presented.

Design and Conduct of Clinical Trials: Report of the Clinical Trials Subcommittee of the International Dry Eye WorkShop (2007)

This report summarizes some universal concepts with regard to clinical trials in general and other issues pertaining to clinical trials specifically tailored to the study of therapeutic intervention in dry eye disease. The report also makes recommendations for logistical design and implementation of such trials. It identifies peculiarities of dry eye disease that complicate clinical trial design, such as the lack of correlation of signs and symptoms, as well as the likelihood of control interventions having a lubricant (placebo) effect. Strategies for environmental trials and controlled adverse environment trials are reviewed.

Methodologies for the Study of Ocular Surface Disease

The ability to obtain reliable results from clinical trials of therapies for ocular allergic disease and dry eye disease is often limited because of inadequate control of variables, such as environment, patient life style, compliance, and individual fluctuations that occur from one assessment visit to another. The controlled allergen challenge (CAC) model of allergic conjunctivitis allows signs and symptoms of the disease to be elicited in a physiologically accurate and reproducible manner. The rigid criteria for subject selection, the controlled allergic reaction, and the standardized and quantified grading systems allow for a reproducible baseline from which statistically and clinically significant differences between formulations can be assessed. Similarly, the controlled adverse environment (CAE) model for dry eye mimics the environmental stimuli that lead to ocular surface drying. Preselected subjects have a reproducible, homogeneous baseline reaction from which the effects of various treatments can be significantly evaluated and compared. CAC and CAE provide accurate means to study highly variable and individual ocular surface disease.

The Effect of Two Novel Lubricant Eye Drops on Tear Film Lipid Layer Thickness in Subjects With Dry Eye Symptoms

PURPOSE

Because the lipid layer of the tear film is recognized as a critical component in dry eye disease, this study was designed to determine if a single eye drop of either Soothe or Systane produces a significant increase in lipid layer thickness (LLT) for subjects reporting symptoms indicative of dry eyes.

METHODS

A double-blind, internally paired study was performed. A custom-built lipid layer interferometer, enabling characterization of lipid layer interference patterns, was used to quantify baseline LLT (OU) of eligible subjects. Inclusion criteria included: 1) presence of dry eye symptoms and 2) baseline LLT < or =75 nm. Subjects (n = 40) received a single eye drop of Soothe in one eye and a single eye drop of Systane in the contralateral eye. After the instillation of each test drop, LLT was reanalyzed for all subjects.

RESULTS

The mean +/- standard error baseline LLT pre-eye drop instillation was 60.0 +/- 1.8 nm for eyes treated with Soothe and 61.5 +/- 1.8 nm for eyes treated with Systane. These means were not significantly different (p > 0.5). The mean LLT for eyes treated with Soothe increased to 124.4 +/- 4.9 nm (p < 0.0001). The mean LLT for eyes treated with Systane increased to 71.3 +/- 2.6 nm (p < 0.0001). The LLT increase from Soothe was significantly greater than that from Systane (p < 0.0001).

CONCLUSIONS

In subjects with symptoms indicative of dry eye states and LLT < or =75 nm, one eye drop of Soothe more than doubled LLT, a 107% mean increase, whereas Systane increased LLT by 16%.

The Contribution of Meibomian Disease to Dry Eye

The tear film lipid layer is the major barrier to evaporation from the ocular surface. A decrease in its thickness or functional integrity may cause evaporative dry eye (EDE). Obstructive meibomian gland dysfunction (MGD) is the most common cause of EDE and occurs as a primary disorder or secondary to acne rosacea, seborrheic or atopic dermatitis, and with cicatrizing conjunctival disorders, such as trachoma, erythema multiforme, and cicatricial pemphigoid. MGD may be an incidental finding in asymptomatic eyes, or it may be responsible for irritative lid symptoms in the absence of dry eye. MGD-dependent EDE is diagnosed on the basis of a defined degree of MGD in a symptomatic patient showing typical ocular surface damage in the absence of an aqueous tear deficiency. When MGD occurs in a background of aqueous tear deficiency (ATD), then an additional evaporative component may assumed, depending on the extent of meibomian obstruction. However, definitive criteria are not yet established. The clinical severity of dry eye is greatest when ATD and EDE occur together, particularly in Sjogren syndrome. A hypothesis is proposed to explain the steps leading to primary, simple MGD and subsequent EDE.